1. Field of the Invention
This invention relates to an electric braking apparatus for a generator, and more particularly to an electric braking apparatus for stopping a brushless excitation system generator by exerting thereon the electric braking action (dynamic braking action).
2. Description of the Prior Art
The brushless excitation system generator, in which the field current decreases following the lowering of the speed of rotation thereof, decreases in its internal induced voltage in a square number of rotations and reduces a short-circuit current, whereby the electric braking apparatus has not been applied to the same as ineffective. The present invention has been designed to solve the above problem.
The electric braking apparatus, as well known, short-circuits a generator circuit to flow a short-circuit current of about a rated current, thereby exerting the braking action on the generator by means of a I.sup.2 R loss thereof. A generator with a slow speed of rotation in a hydroelectric power station or the like possesses a large rotational energy because of the parts, so that the mechanical brake, which uses brake shoes causes a large quantity of heat, thereby creating a problem in maintenance of the same not only because the braking effect is slow but also because the brake shoes wear out. Hence, the idea of using the electric brake for the generator normally and using the mechanical brake only for the emergency stopping, has been taken in a close-up.
Next, an explanation will be given about an example of the electric braking apparatus of the conventional thyristor excitation system generator with reference to FIG. 1, in which reference numeral 1 designates a generator, 1A designates a field (winding) thereof, 2 designates a circuit breaker for the electric brake, 3 designates a circuit breaker for the generator, 4 designates a main transformer, 5 designates a transformer for meters, 6 designates an exciting transformer, 7 designates an automatic voltage regulator (AVR), 8 designates a firing circuit (GPG: Gate Pulse Generator), 9 designates a thyristor bridge, 10 designates a field circuit breaker, 11 designates a field discharging resistor, 11A designates a contactor closing in association with open (cutoff) of the field circuit breaker 10, 12 designates a contactor for the initial excitation and electric braking action, and 13 designates a diode bridge for the initial excitation and electric braking action.
The circuit breaker 3 for the generator and the field circuit breaker 10 are turned off by a stop instruction to the generator and the contactor 11A is turned on to deenergize a field current in the field 1A at the generator 1, so that, when the existing generator voltage is equal to the residual voltage, the circuit breaker 2 for the electric braking apparatus is closed. Then, the contactor 12 for the electric braking is on so as to excite the field 1A of the generator 1 through diode bridge 13 by flowing in the field 1A a short circuit current of about the rated current of the generator, thereby exerting the electric braking action on the generator 1.
A current flowing during the electric braking action is given in the following equation: ##EQU1## where I: a current flowing during the electric braking action,
S: Slip, PA1 V: Generator Voltage, PA1 R: Armature Resistance, and PA1 xd: Synchronous Reactance.
The conventional thyristor excitation system generator ensures an exciting source even when the speed of rotation of generator 1 is lowered, thereby enabling the current expressed in the above equation to continuously flow. Hence, the electric braking effect is kept in effect until the generator stops, whereby such the system has hitherto been adopted.
Meanwhile, an embodiment of an excitation circuit for the conventional brushless excitation system generator is shown in FIG. 2, in which reference numeral 1 designates a generator, 1A designates field winding thereof, 3 designates a circuit breaker for the generator, 4 designates a main transformer, 5 designates a potential transformer, 6 designates an exciting transformer, 7 designates an automatic voltage regulator (AVR), 8 designates a firing circuit (GPG), 9 designates a rectifying circuit comprising a thyristor bridge, 10 designates a field circuit breaker, 14 designates a brushless AC exciter connected directly with the generator 1, 15 designates a rotating rectifier, 16 designates an initial exciting contactor, 17 designates a reverse current blocking diode, 18 designates an adjusting resistor, and 19 designates a battery.
This kind of conventional brushless excitation system generator does not need to use slip rings or brushes thereby being widely used because of the advantage of facilitating the maintenance. However, since the brushless AC exciter 14 is connected directly with the generator 1, the excitation capacity of the exciter 14 lowers as the speed of rotations of the generator 1 decreases, thereby creating the problem in that there is no effect of the electric braking action.